Ceramic fibers are recognized as having utility in a variety of industrial composites. Composite materials are desired combinations of two or more constituent materials to form a bonded structure that has mechanical, physical and chemical advantages over each individual constituent. In particular, ceramic matrix composites contain a matrix component that may be a ceramic oxide and a reinforcing material like a ceramic fiber. Some of the various applications of these composites are cutting tools, armor, engine parts, electronic devices, catalysts, and medical implants.
Electrolyte tiles in molten carbonate fuel cell prepared from gamma lithium aluminate powder and lithium carbonate/potassium carbonate electrolyte could be improved by incorporating suitable reinforcing fibers in the tiles. A typical tile is about 1 m.sup.2 in area and from 0.5-2 mm in thickness. Reduction in tile thickness is desirable in order to reduce electrical resistance. This can be achieved by the addition of reinforcement fibers to increase the strength and durability of the tile structure and the overall lifetime of the cell. Such reinforcing fibers must not only have high strength but the fibers must also be compatible with the matrix material, and be thermally stable at temperatures up to at least 650.degree. C. under the molten carbonate fuel cell environment. Alumina fibers have been tested but formation of lithium aluminate occurred. Ideally, lithium aluminate fibers are the best choice for the molten carbonate fuel cell electrolyte tile. Other applications may potentially require other lithium aluminum oxide fibers such as lithium pentaaluminate, lithium dialuminate, or other lithium metal oxide fibers.
Methods developed for the production of ceramic fibers include spinning, colloidal evaporation, vapor deposition, and drawing from a melt. Non-oxide fibers like silicon carbide fibers have been prepared by these processes but limitations have been observed when applied to compositions containing glass-forming oxides. An example is the production of ceramic refractory fibers like alumina-silica, alumina, and zirconia by a blowing method wherein a stream of the molten material is subjected to a jet of air or steam. The disadvantages of this method are the requirement for a huge capital investment in specialized plant machinery and the phase separation that may occur from rapidly cooling the drawn fiber to prevent devitrification or crystallization. In the solution spinning process currently used for the production of alumina fibers, control over the form of the fibers, such as fiber diameter, is done by spinning the alumina fibers at low temperature from a melt of pure raw materials containing not more than 60-65% alumina followed by heat treatment above 1000.degree. C.
The synthesis of polycrystalline lithium metal oxide fibers such as lithium aluminate fibers has been investigated but only a few successful processes have been reported. One of the major difficulties in the preparation of lithium aluminate fibers by drawing from a melt is the loss of lithium oxide due to evolution at elevated temperatures. Therefore, a lower temperature or shorter time would be more desirable for the synthesis of lithium aluminate fibers, conditions which do not generally lead to proper densification or proper phase of the lithium aluminate.
Watanabe and his co-workers disclosed in Japanese patent application 63-260812 a method for the preparation of long, rod-shaped .beta.-LiAlO.sub.2 crystals in the Journal of the American Ceramic Society, Vol. 70:10, C-268-269 (1987). Raw materials were LiOH.H.sub.2 O:.gamma.-Al.sub.2 O.sub.3 :NaOH in mole ratios of 4:1:4. Columnar test pieces were shaped from these mixed powders under 9.8 MPa pressure, fired at 600.degree. C. for three hours, cooled, and dipped into water for 24 hours to yield rod-shaped .beta.-LiAlO.sub.2 crystals that were 1.5 .mu.m in diameter and 10 to 15 .mu.m long. Examples cited showed the fiber properties of these crystals obtainable under various powder compositions and calcination conditions. The problems with this approach include the potential conversion of .beta.-LiAlO.sub.2 to other phases in the fuel cell. The small size of the crystal also limits its value as a reinforcement.
In International Patent Application WO 90/04859, Smith and Kucera disclosed a method of preparing ceramic oxide fibers by spraying a slurry of the ceramic material perpendicularly into an ambient stream of air or other gas and then heating the resultant green fibers to remove the binders. Examples cited were lithium ferrite and lithium manganite fibers which are potential electrode materials in molten carbonate fuel cells. Application of this technology to the production of lithium aluminate fibers and subsequent benefits were demonstrated by P. M. Brown in the "Physical Property Optimization of Lithium Aluminates for Fabrication of Molten Carbonate Fuel Cell Matrices" which was presented at the 1990 Fuel Cell Seminar. The resultant molten carbonate fuel cell electrolyte tiles produced with inclusion of the fibers increased the tile strength. The need exists for improved fibers to yield further improvements in the electrolyte tile strength or in other applications.
Charles C. Fain in U.S. Pat. Nos. 4,798,815 and 4,820,664 disclosed the piggyback method of making ceramic fibers by wetting special carbon fiber micromolds, which are non-ceramic fibers having elongated cavities, with a wet chemical precursor and subsequently heating the wetted micromolds to produce ceramic fibers of various compositions. Although the `piggyback` method of producing polycrystalline oxide fibers is generally useful for producing otherwise difficult to produce fibers, U.S. Pat. Nos. 4,798,815 and 4,820,664 do not disclose the processing times, temperatures, procedures, or carrier media necessary for preparation of high quality dense lithium metal oxide fibers without loss of lithium through lithium oxide evolution which occurs at temperatures higher than 1000.degree. C.